The easiest method is to clone this codebase using github.
Then, inside of the main directory create a config.yaml file, and add the text DISCORD_TOKEN: <your-token-here>. If you don't know how to get a discord token, you can go here, create an app, make a bot account for it, and use the token generated for that bot.
Make sure you bundle in your terminal inside of the directory for your bot before you, again in your terminal, use ruby run.rb to start the bot up. (Make sure to invite it to your server!)
This bot is built using the ruby gem discordrb, where you can find a lot of information about setting up your own bot and building new functionality into it!
And a lot of the features of this bot are modeled on those in the Scryfall Discord bot, though I lovingly ripped off some code from their original version which is both written in javascript (mine is in ruby, so some translation work was involved) and can be found here. EDIT: As of 2020, we're all willing to admit that the scryfall bot can and should just be doing this on its own. I've retained the ruby code that I translated from the scryfall bot, but I've commented it out so that josh-bot has to do less.